392 



ANIMAL RESPIRATION 



shelly plates, and exhibiting below a large flat muscular foot, by 

 means of which creeping is effected. Overhanging the foot, 

 and running right round the animal, is a flap-like mantle-skirt, 



the groove under which 

 is known as the mantle- 

 cavity. This shelters on 

 each side a varying num- 

 ber of gills (fig. 521), 

 s each of which consists 

 of a stalk, on either side 

 of which are arranged a 



INTESTINAL 

 OPENING 



Fig. 521. Mail-Shell (Chiton] seen from above (A) and below (B). 

 Note in B the numerous gills on each side 



large number of small 

 flat plates. These col- 

 lectively present a very 

 large surface to the surrounding water, and as the gills are of 

 delicate texture it is easy for the abundant blood which they 



contain to get rid of its 

 waste carbonic acid gas in 

 exchange for oxygen. 



FIN 



Fig. 522. Dissection of a Cuttle-Fish (Sepia) from the back, to 

 show interior of gill-cavity. The muscular mantle has been cut 

 through and turned over to the right, and the tentacles have been 

 cut short: X X (on mantle), two projections which "button" into 

 two corresponding pits (X X) while water is being ejected through 

 funnel; K K, kidney openings; Lap. Intestinal aperture. Course 

 of breathing-water shown by arrows. 



HEAD-FOOTED MOL- 

 LUSCS (CEPHALOPODA) 



A Cuttle -Fish (Sepia, 

 &c.) or Squid (Loligo, &c.) 

 is far more specialized than 

 a Mail -Shell as regards 

 breathing organs. On the 

 hinder side of its long body 

 we can easily find a large 

 gill-cavity, opening below 

 by a large slit, and probably 

 equivalent to the posterior 

 part of the mantle-cavity in 

 the Mail-Shell, greatly en- 

 larged and deepened. Two 

 very large plume-like gills 

 are contained in the cavity 

 (fig. 522), and these pro- 

 bably represent one of the 

 last few pairs of a Mail- 



